Materials that have improved fracture toughness, such as cast iron materials or cast steel materials, are widely employed for the components, the constructional parts and the movable members of various apparatuses and machines. For example, for the stages, the support members and the driver components of precision machinery for which high accuracy is required, casting molds, plastic injection molds and aluminum die casting molds, a cast steel material that has a low thermal expansion coefficient and stable properties is a prerequisite. Especially for precision machines employed in cold areas or high temperature areas, the affect of thermal expansion should be minimized. Therefore, a material for which martensitic transformation has been at least partially completed is requested.
In order to respond to such a technological request, cast iron having superior wear resistance and corrosion resistance, and that is appropriate for use as a cylinder liner, is disclosed in patent literature 1. For satisfying such an intended purpose, this cast iron contains high densities of phosphorus (P) and boron (B), and a surface hardened layer (steadite) is dispersed. However, since martensitic transformation is not performed for this material, it is understood that acquisition of a satisfactory hardness, such as HRC 30 or greater, is difficult for the cast iron. Patent literature 2 discloses cast iron that exhibits superior heat resistance and corrosion resistance at high temperatures in corrosive gas atmospheres, and also demonstrates a superior wear resistance. This cast iron contains a large amount of chromium Cr, 30 to 50 mass %, for example, and is very hard and brittle. According to the description in patent literature 2, the cast iron is employed for the fire gate of an incinerator, as an example, and is not an appropriate material for forming machinery components and molds. In order to increase the hardness of the base metal of the cast iron, a conventional, common high-temperature heat-treatment (quenching) is required.
In patent literature 3, a martensitic cast steel material is disclosed, in which nickel, Ni, manganese, Mn, silicon, Si, and carbon, C, are contained at predetermined ratios and the remaining part consists of iron, Fe, and incidental impurities, and for which the martensitic transformation start temperature is near a room temperature and the martensitic transformation finish temperature is below freezing, and a manufacturing method is also disclosed, for melting such a martensitic cast steel material to produce steel casting products that are shaped almost as are final products. According to the martensitic steel casting product manufacturing method that is disclosed, a sub-zero treatment (deep freezing) is performed for such obtained steel casting products at 0° C. to −200° C., and thereafter, a tempering process is performed at a required temperature. Then, either the sub-zero treatment or the tempering process is repeated, as needed, and a finishing process is performed. Since the hardness of a martensitic steel casting product can be increased simply by performing the sub-zero treatment, thus obtained martensitic steel casting product is an appropriate material to use for casting elevator sheaves, various industrial machinery wheels, etc. However, improved corrosion resistance is still required, so that the cast steel material can also be used in wet environments. It has long been known that adding chromium, Cr, to ferrous materials is an effective means by which to improve corrosion resistance. However, when chromium, Cr, is added to a cast iron or cast steel material, hard and brittle carbide is generated that, as the structural material of a machine, would cause the deterioration of important properties, such as ductility and toughness, so that as yet, such a steel casting product has not been put to use.
[Citation List]
Patent Literature
    [PTL 1]
Japanese Unexamined Patent Publication No. 2006-206986    [PTL 2]
Japanese Unexamined Patent Publication No. 2004-270002    [PTL 3]
Japanese Unexamined Patent Publication No. 2006-104573